1. Field of the Invention
The present invention relates to a liquid crystal device which can be used for flat displays for portable information terminals, personal computers, wordprocessors, amusement apparatuses, television sets, etc. viewed by a plurality of viewers, display boards employing a shutter effect, windows, doors, walls, or the like, and a method for fabricating the same.
2. Description of the Related Art
As Liquid crystal devices such as liquid crystal display devices employing the electro-optic effect, the twisted nematic (TN) type, the super twisted nematic (STN) type, and the like using nematic liquid crystal have already been commercialized. These types of liquid crystal devices require polarizing plates and aligning treatment. These liquid crystal devices, such as liquid crystal display devices, have a pretilt angle in an initial orientation state, so that liquid crystal molecules raise in the direction of the pretilt angle when the liquid crystal cell is applied with a voltage, as shown in FIG. 22B. Accordingly, when such a liquid crystal display device is viewed from different viewing angles A and B, the apparent refractive index of the liquid crystal molecules differs depending on the viewing angle, changing the contrast of the display or even inverting the contrast at the gray scale display level depending on the viewing angle. This significantly lowers the display quality.
On the other hand, some liquid crystal devices utilize scattering phenomena of liquid crystal, and do not use polarizing plates. These devices use a dynamic scattering (DS) effect and a phase change (PC) effect.
In recent years, a method for electrically controlling the transparent and opaque states of liquid crystal by using the birefringence of liquid crystal has been proposed. This method requires neither polarizing plates nor aligning treatment. According to this method, basically, the ordinary ray refractive index of liquid crystal molecules and the refractive index of a support medium are set identical. The transparent state is presented when the liquid crystal molecules are aligned by applying a voltage. The opaque state caused by light-scattering is presented when the liquid crystal molecules are not aligned, i.e., when no voltage is applied.
The above method is disclosed, for example, in Japanese Laid-Open National Patent Publication No. 58-501631 where liquid crystal is contained in a polymer capsule, and in Japanese Laid-Open National Patent Publication No. 61-502128 where liquid crystal and a photocurable resin or a thermosetting resin are mixed and the resin in the mixture is cured to separate the liquid crystal from the resin, thereby forming liquid crystal droplets in the resin. The liquid crystal devices obtained by these methods are called "polymer dispersed liquid crystal display devices".
Moreover, methods for improving the viewing angle characteristic of a liquid crystal cell by employing polarizing plates are disclosed in Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928, where the above-described polymer dispersed liquid crystal display device is sandwiched between polarizing plates disposed to cross each other at right angles. This device greatly improves the viewing angle characteristic. However, since this device utilizes, in principle, depolarization caused by light scattering, the brightness of this type of device is half as low as that obtained by a TN mode device, and therefore the usability is low.
Further, another method for improving the viewing angle characteristic is disclosed in Japanese Laid-Open Patent Publication No. 5-27242, where the orientation of liquid crystal is disturbed by a polymer wall and a protrusion so as to form randomly liquid crystal domains. In this method, however, since the domains are formed randomly and a polymer material is present in pixel portions, the light transmittance at the time of no voltage application is lowered. Moreover, disclination lines randomly arise at the boundary of the liquid crystal domains and do not disappear even when a voltage is applied. This results in degradation of the black level at the time of voltage application. Due to the above reasons, the contrast of this type of the liquid crystal device is lowered.
Yet another method for improving the viewing angle characteristic is proposed by Japanese Laid-Open Patent Publication No. 6-301015 and Japanese Patent Application No. 5-199285 assigned to the same assignee of the present application, where liquid crystal molecules are aligned axial-symmetrically, for example, radially or concentrically (tangentially).
The above-described liquid crystal devices significantly improve the viewing angle characteristic, as described above. However, in these liquid crystal devices, the orientation of the liquid crystal may be disturbed due to undefined factors such as remainders of resist and scratches on the substrate. This causes the symmetry axis of the orientation of the liquid crystal molecules to incline or displace as shown in FIG. 23. This figure is a diagram of a liquid crystal device observed with a polarizing microscope. In such a case, when the liquid crystal device is viewed from different viewing angles, the area of a region of which orientation corresponds to a certain viewing direction (a black region) in one pixel becomes greater compared with other pixels. As a result, the average transmittance of the pixel differs from the transmittance of other pixels. This is observed by the viewer as roughness of display. Accordingly, in the above liquid crystal devices, the symmetry axis for the orientation of liquid crystal molecules should be strictly controlled.
Moreover, it is necessary to stabilize the axial-symmetric orientation state for easy fabrication of liquid crystal devices. The axial-symmetric orientation is mainly disturbed by the non-uniformity of surface free energy on the substrate.